Fluoroelastomers comprising a fluoromonomer unit as a recurring unit such as tetrafluoroethylene (TFE) are used widely as a sealing material to be used under strict environment since they exhibit excellent chemical resistance, solvent resistance and heat resistance. Particularly in the fields of aviation and space industries, semi-conductor production apparatuses and chemical plants, which are major applications of the fluoroelastomers, heat resistance of sealing at 300° C. or more is demanded.
Such a crosslinked product having excellent heat resistance is obtained by firstly preparing a nitrile group-containing fluoroelastomer having nitrile group in a molecular chain as a cure site by copolymerizing perfluoro(vinyl ether) (CNVE) having nitrile group with TFE and perfluoro(alkyl vinyl ether) (PAVE) and then crosslinking the fluoroelastomer by triazine crosslinking with organotin, oxazole crosslinking with a bisaminophenol compound or tetraamine compound or imidazole crosslinking, as described, for example, in JP-A-58-152041 and JP-A-59-109546.
However CNVE for introducing nitrile group as a cure site requires a long synthesis step and is very expensive, and also the nitrile group in the obtained nitrile group-containing fluoroelastomer is unstable. Further a mechanical strength and sealing property at high temperature (compression set) of the obtained crosslinked product are insufficient.
The inventors of the present invention have made various studies with respect to conventional nitrile group-containing fluoroelastomers and assumed that a reason why sufficient properties were not obtained was the fact that nitrile cure site group is present only as a pendant of a molecular chain. Thus the inventors have studied introduction of nitrile group into an end of the molecular chain. However in a conventional method described in Polymer Journal, Vol. 17, No. 1, pp 253 to 263, in which a solid dehydrating agent such as phosphorus pentoxide is used (Formula 10 at page 261), it is possible to carry out conversion of an end group of liquid fluorooil into nitrile group but a solid fluoroelastomer cannot be dehydrated uniformly and the dehydrating agent remains in the obtained product. Therefore an intended solid fluoroelastomer having nitrile group at the end could not be obtained.
An object of the present invention is to provide a solid fluoroelastomer having, at the end, nitrile group functioning as a cure site, and an economical preparation process thereof.
Another object of the present invention is to provide a novel synthesis process for converting amide group into nitrile group, and a dehydrating agent to be used therefor.